1. Field of the Invention
The present invention relates to a plasma generating apparatus and method for an extreme-ultraviolet laser.
2. Description of the Related Art
Since the invention of a laser, efforts to shorten wavelengths of lasers have been continued, and as the wavelengths of lasers have been shortened, fields of laser application have been gradually widened. Recently, in particular, extreme-ultraviolet lasers (which are often called an "X-ray laser") whose wavelength is less than 100 nm, have been extensively studied. To shorten the wavelengths of lasers, a great exciting power no less than fourth power of the reciprocal of a laser wavelength is required. This presents many physical and technical problems that must be solved before an extreme-ultraviolet laser is realized. One promising extreme-ultraviolet laser is a recombination laser. This laser attempts to achieve an extreme-ultraviolet amplification by rapidly cooling a high temperature plasma. The most rapid cooling method is an adiabatic expansion cooling, if the size of the initial hot plasma is small enough. This method is based on the principle that the temperature drop takes place when gases are subjected to free expansion because thermal motion of particles are changed into translational motion, accompanying the density reduction by the expansion. To achieve sufficient amplification of an extreme-ultraviolet laser, the degree of population inversion achieved in nonequilibrium state must be great. This requires that the temperature of a plasma must be dropped faster than the speed of progress toward the thermal equilibrium. That is, a rapid expansion is required.
A greater density reduction is accomplished when a cylindrical plasma expands radially than when a plane plasma expands linearly. Accordingly, greater amplification power of an extreme-ultraviolet laser will be expected with a cylindrical plasma. The difference anticipated with the plane plasma and the cylindrical plasma has been experimentally confirmed (T. Tomie et al., X-ray lasers 1990, ed. by G. Tallents, Institute of Physics Conference Series 116, 1990). The cylindrical plasma from which a rapid expansion is expected can be generated by irradiating a thin fiber by an exciting laser. It was experimentally confirmed that 18 nanometer extreme-ultraviolet light was amplified using a seven micrometer diameter carbon fiber as a target (C. Chenais-Popovics, et al., Physical Review Letters, volume 59, number 19 (1987)).
Although C. Chenais-Popovics et al., reported a moderate degree of amplification of extreme ultraviolet light of 18 nanometer wavelength using a seven micrometer diameter by eight millimeter long carbon fiber as a target, several centimeter long plasma is required to achieve a sufficient amplification and to realize a practical extreme-ultraviolet laser of sufficient energy. Since light travels in straight lines, the plasma must also maintain the straightness in order to amplify extreme-ultraviolet light rays. However, it is inevitable for a thin, seven micrometer diameter fiber to bend, and hence, it is almost impossible to generate more than two centimeter long straight plasma by a fiber target. Furthermore, it is impossible for a thin fiber to absorb exciting laser energy with high efficiency: in experiments, absorption fractions less than only 10% have been achieved.